Threaded connections for use in connecting adjacent sections of a pipe string are well known. In the oil and gas industry, threaded connections have long been used in a wide variety of applications, such as connecting adjacent sections of a well Casing, a drill string, or a pipeline.
As seen in FIG. 1, the drill string used for drilling into underground oil and gas reservoirs includes at least a drillpipe string 10, a drill collar string 12, and a drill bit 14. The drill collar string 12 comprises the major part of the bottom-hole assembly ("BHA") which is the lower section of the drill string between the drillpipe string 10 and the drill bit 14. Additional components of the BHA (not shown) may include other equipment such as measurement-while-drilling ("MWD") tools, the bit sub, drill-stem subs, crossover subs, shock subs, jars, stabilizers, reamers, fishing tools, and nonmagnetic collars. Drilling fluid is pumped down through the bore of the drillpipe string 10 and drill collar string 12 and through nozzles (not shown) located on the drill bit 14. The drilling fluid then becomes mixed with the rock dislodged by the drill bit 14 and returns to the surface through the annulus 15 between the borehole wall and drill string.
The drill collar string 12 is composed primarily of a series of heavy-walled pipes called drill collars and is used primarily to provide enough weight for producing the desired compressive load at the drill bit 14. Also, the drill collar string 12 may provide some additional weight to ensure that the drillpipe string 10 remains in tension. As a result of this weight, the lower portion of the drill collar string 12 experiences significant compressive stresses, while the upper portion of the drill collar string 12 experiences tensile stresses. Furthermore, when the drill collar string 12 rotates bending occurs at the drill collar joints or connections 18 (see FIG. 1A) which are the weakest points along the drill collar string 12. Consequently, fatigue failure resulting from this combination of compressive, tensile, and bending stresses typically occurs at the drill collar connections 18.
Drill collars 16 are normally manufactured in an average length of about 30 feet. Owing to the large wall thickness of the pipe, drill collars 16 are not provided with tool joints, which for drill pipe are typically stiffer and stronger than the pipe body itself, for connecting the drill collars 16. Instead, as seen in FIG. 1A, they are connected using a threaded connection 18 which comprises pin member 20 and box member 22, each of which is integrally machined from the pipe. The drill collar connection 18 is "made-up" by rotating the pin member 20 into the box member 22 up to the desired torque to form a metal-to-metal seal between adjacent drill collars at the shoulder 24 located at the base of the pin member 20 and the face end 28 of the corresponding box member 22. This make-up torque should be sufficient to prevent the seal from opening up under the bending loads produced as the drill string rotates.
Drill collar connections 18 are subjected to torsional stresses arising when the connection is made-up as well as compressive and bending stresses while drilling, discussed above. Consequently, in order to reduce premature fatigue failure, the drill collar connections 18 must be manufactured to withstand these various stresses while maintaining a fluid-tight seal under considerable internal pressure. The American Petroleum Institute ("API") provides connection specifications for assorted drill collar sizes based on the desired drill collar outside diameter ("OD"), inside diameter ("ID"), and bending strength ratio ("BSR"). API drill collar guidelines, for example, are identified in Recommended Practice for Drill Stem Design and Operating Limits ("RRP7G"), 14th Edition, Aug. 1, 1990, Section 3, while manufacturing specifications are provided in Specification for Rotary Drilling Equipment ("SPEC 7"), 37th Edition, Aug. 1, 1990, Sections 6 and 9 and Appendix I. API specifications are generally adhered to throughout the oil and gas drilling industry worldwide. However, a departure from these specifications is permissible where a significant improvement in drilling equipment performance can be realized using different specifications. Th design specifications of the inventive connections represent such a departure.
There is a natural variation between the strengths of the pin member 20 and box member 22 which occurs as their relative sizes vary along the axis of the connection. The pin member 20 may be so large that the portion of the box member 22 near the pin shoulder 24 would be so thin that it could not support the pin member 20 in bending. Farther from the shoulder 24, however, the tapered box wall thickens to a point where it can support the pin member 20 in bending. This point is usually in the region of the pin nose 23 (see FIG. 1A). It is generally believed by those skilled in the art that this region provides a likely site for fatigue failure with continual flexing of the drill collar connection 18, resulting in the box member 22 eventually failing in that region. Accordingly, to avert the early onset of such fatigue failure in the connection 18, the drilling industry has identified a generally preferred balance between box and pin strength to yield a "balanced connection". Using conventional drill collar design principles, the API specifications discussed above can assist a person skilled in the art in designing such a "balanced connection".
Bending strength ratio is a number typically used by those skilled in the art to quantify the balance between box and pin strength. BSR is the ratio of the box member modulus (Z.sub.B) to the pin member modulus (Z.sub.P). Since the member modulus (Z) is the measure of the member's capacity to resist any bending moment to which it may be subjected, the BSR is a number descriptive of the relative capacity of the box member 22 versus the pin member 20 to resist bending fatigue failures. Therefore, a higher BSR leads to a stronger box member and a correspondingly weaker pin member.
API recommended practice RP7G suggests that a connection having a BSR of 2.50 is generally accepted as an average balanced connection under average drilling conditions. However, RP7G further suggests that the acceptable BSR range may vary from 1.90 to 3.20 depending upon the drilling conditions. Nonetheless, certain published guidelines routinely used throughout the oil and gas drilling industry, such as the Drilco Drilling Assembly Handbook, generally recommend drill collar connections having a BSR in the range of 2.50 to 2.75 for obtaining well-balanced connections with optimum fatigue-life.
BSR is calculated in accordance with API RP7G (Appendix A.10) using the following relations. EQU BSR=(D.sup.4 -b.sup.4)R/[D(R.sup.4 -d.sup.4)] (1)
where "D" is the drill collar's OD, "d" is the drill collar's ID, "b" is the thread root diameter of the box threads at the end of the pin (when connection is fully assembled), and "R" is the thread root diameter of pin threads 3/4 inch from shoulder of pin.
In equation (1), "b" and "R" may be calculated from the following equations: ##EQU1## wherein: H=Thread height not truncated
f.sub.rn =Root truncation (inches) PA1 C=pitch diameter (inches), measured at 0.625 inches from shoulder of pin PA1 L.sub.pc -0.625=length from pitch diameter point to end of pin (inches) PA1 tpr=taper, inches per foot on diameter
As shown by the above relationships, BSR depends on a number of variables including the drill collar's OD and ID, the pitch diameter and taper of the pin member 20 and box member 22, as well as pin length of certain thread dimensions. Normally the drill collar OD and ID are selected based on various operational considerations. Also, the thread dimensions are typically dictated by standard thread forms, such as the API V-0.038R, V-0.040, or V-0.050, preferred in the industry. Consequently, the desired BSR for the selected drill collar OD/ID and thread form could be obtained by machining the pin member 20 and box member 22 to yield the appropriate pitch diameter, taper, and pin length.
Currently, the drilling industry prefers connection designs using the boreback box design originally disclosed in U.S. Pat. No. 2,745,685 issued May 15, 1956 to Moore. This box design is preferred as a result of the relative ease in machining the box member. Also, the drilling industry has typically preferred those connection designs which favor preserving the pin member 20 relative to the box member 22. This preference for greater pin member strength relative to the box member 22 may have arisen because a crack before parting in the box member is more easily detected at the surface by a drop in mud pressure than is a comparable crack in the pin member. There is substantial incentive to detecting cracks before they propagate and cause separation because a downhole failure can be very difficult and costly to retrieve. However, using modern inspection practices, the likelihood of detecting pin member fatigue cracks has been significantly improved. Consequently, in the opinion of the inventors, it is presently possible to achieve a more cost-effective drill collar connection, in terms of both failure frequency and correction costs, by using connection designs which provide BSR values with improved box member strength at the expense of some pin member strength, even though such BSR values are contrary to the accepted practice of the API and others skilled in the art.
As mentioned previously, the oil and gas drilling industry has typically designed or selected drill collar connections having BSRs between about 2.25 and about 2.75 to avert early fatigue failures induced by bending stresses. Nevertheless, bending stresses are believed by the inventors to be the leading contributor to fatigue failures in drill collars and other BHA components which continue to plague the drilling industry. In the inventors' opinion, the overwhelming majority of these failures occur in the BHA connections as fatigue cracks, with a predominance of those failures occurring in the box member. Such failures can result in the loss of expensive tools located in the drill collars, lost rig time, and increased drilling costs arising from additional drill string trips.
Accordingly, there is a need for a threaded connection having improved fatigue-life for use in a drill collar string which is subject to bending stresses in combination with tensile and compressive stresses.